(1) Field of the Invention
The present invention relates to, for example, in a WDM (Wavelength Division Multiplexing) linear repeating transmission system (which will hereinafter be referred to as a WDM transmission system), a dispersion compensation quantity (value) setting method, receiving terminal station and wavelength-multiplexing optical transmission system, suitable for the compensation for waveform distortions occurring due to nonlinear optical effects and the compensation for chromatic dispersions.
(2) Description of the Related Art
In general, for the transmission of wavelength-multiplexed light, a WDM transmission system (WDM optical communication system) using an optical amplifier is effective in the reduction of circuit cost. In this WDM transmission system, the enhancement of bit rate and the improvement of wavelength density are indispensable to the expansion of transmission capacity.
(i) Expansion of Transmission Capacity
As well known, the methods of expanding the transmission capacity are classified into a method of increasing the number of wavelengths of transmitted light within a specific wavelength band (densification of number of wavelengths) and a method of enlarging a wavelength band to increase the number of wavelengths (enlargement of wavelength band). The densification of number of wavelengths causes the interference among a plurality of transmitted lights close in wavelength to each other and leads to marvelous nonlinear optical effects of an optical fiber. Accordingly, on the other side of the expansion of the transmission capacity, waveform distortions occurs and difficulty is experienced in making long-distance transmissions.
On the other hand, on the enlargement of wavelength band, limitation is imposed due to the amplification characteristics of devices themselves, such as an EDFA (Erbium-Doped Optical Fiber Amplifier). That is, since limitation is imposed on the device amplified wavelength bandwidth, difficulty is encountered in enlarging the wavelength band.
(ii) Long-Distance Transmission
In general, in a WDM transmission system using an optical amplifier, due to the nonlinear optical effects in a transmission line, simultaneous with a waveform degradation stemming from the self phase modulation (which will hereinafter be referred to as a self phase modulation SPM) which is a phase modulation occurring in accordance with an intensity variation of a transmission channel itself, a waveform degradation develops due to the cross phase modulation (which will hereinafter be referred to as a cross phase modulation XPM) which is a phase modulation occurring in accordance with an intensity variation of a channel different in wavelength. Coupled with the dispersion, each phase modulation component is converted into a waveform distortion. For this reason, the optical signal transmission distance becomes longer by suppressing the dispersion and the nonlinear optical effects.
(iii) Dispersion Compensator
For the suppression of the dispersion, a dispersion compensator is provided in an optical fiber transmission line and, in consideration of the non-uniformity of the dispersion quantity, it is provided in the interior of a repeating node (repeater, repeater station, repeating unit or optical amplification repeating unit) or in an optical fiber transmission line. Moreover, the “position and the degree of the compensation” scalled dispersion compensation management, and various examinations have been made on the location and others of the dispersion compensator.
The repeating nodes can be located at an approximately equal interval in a case in which the optical fiber transmission line is laid on the bottom of the sea. On the other hand, in a case in which an optical fiber transmission line is on land, in view of cost, difficulty is experienced in making the interval between the repeating nodes constant, and the dispersion compensation quantity needed for each repeating span (repeating zone) is not always made constant.
(iv) Linear Repeating Transmission System
In the case of carrying out the dispersion compensation in the WDM transmission system, a manager determines a location of a dispersion compensator and a dispersion compensation quantity thereof in consideration of an optical fiber transmission line condition (for example, the dispersion of an optical fiber and the influence of the nonlinear optical effect occurring in an optical fiber transmission line). In general, the location of a dispersion compensator and the dispersion compensation quantity thereof are determined through repeating transmission simulations in accordance with various types of characteristics of a given optical fiber transmission line.
In this connection, as the technique of improving the waveform distortion in an optical transmission system, there are been proposed a method of compensating for the influence of the phase modulation (for example, Patent Document 1).
A transmission system disclosed in this Patent Document 1 is prechirped through the use of an amplitude modulation replica applied to each of other channels or a replica passing through a low-pass filter on a transmitter side for each individual channel in a WDM transmission system which transmits traffic amplitude-modulated to develop significant cross phase modulation XPM. For the compensation for the self phase modulation SPM, for each channel, it is also acceptable to additionally or substitutively employ the prechirp using an amplitude modulation replica applied to that channel. This eliminates at least a portion of the adverse influence of the cross phase modulation XPM and/or the self phase modulation SPM, thus providing a transmission system with a compensation function.
In addition, Patent Document 2 discloses an optical transmission system in which the transmission side is equipped with a dispersion assigning unit to assign chromatic dispersions different from each other to a signal light in an adjacent channel (wavelength) and the reception side is provided with a dispersion compensator to compensate for the chromatic dispersions assigned to the signal light in each channel on the transmission side and for the chromatic dispersion in an optical fiber transmission line. This can suppress the crosstalk from the adjacent channel with effect and achieve the effective utilization of a band by making the wavelength spacing closer.
Patent Document 1: Japanese Patent Laid-Open No. HEI11-149064
Patent Document 2: Japanese Patent Laid-Open No. 2000-183815
However, in the WDM transmission system using an optical amplifier, the transmitted light undergoes the phase modulation due to the self phase modulation SPM and the cross phase modulation XPM. Moreover, for example, if the number of multiplexes is taken as i (i denotes a natural number), the j-th (j represents a natural number smaller than i) wavelength light (channel #j) develops a waveform distortion arising from the synergy of the factors of the self phase modulation SPM stemming from the wavelength light j itself, the cross phase modulation XPM between this self phase modulation SPM and the (i−1) wavelength lights other than the wavelength light j and the dispersions.
Furthermore, various methods of individually carrying out the effective compensation on the self phase modulation SPM and the effective compensation on the cross phase compensation XPM have been known, whereas a method of carrying out both these compensations simultaneously is difficult in design.
Still furthermore, although, for suppressing the occurrence of the crosstalk (XPM between different channels), for example, the location of a dispersion compensator is determined through simulations or the like conducted in advance, the sufficient optimization requires detailed examination with high accuracy and, in a case in which the actual transmission environment differs from the condition assumed at the design, the transmission quality lowers with respect to that at the design.
In addition to this, the realization of the optimization becomes difficult for taking into consideration the differences among the performances of the optical fibers, optical amplifiers and others and, hence, for example, a manager decreases the number of wavelengths or shortens the transmission distance, which can lead to the degradation of the performances.
Meanwhile, the aforesaid Japanese Patent 1 can compensate for the XPM introduced into a first span after a transmitter but it cannot compensate for the XPM occurring due to the nonlinear optical effects in the second and subsequent spans as a result of the light intensity amplified in a relevant amplifier. In addition, difficulty is encountered in setting the dispersion compensation quantity variably.
Moreover, the aforesaid Japanese Patent 2 relates to a technique of reducing the peal intensity by assigning the prechirp to a signal light in each channel, and is for suppressing the influence of the nonlinear optical effects in an optical fiber transmission line. Therefore, it does not provide a mode taking both the self phase modulation SPM and cross phase modulation XPM into consideration.